Formation of copper—iron sulfide minerals during corrosion of artifacts and possible implications for pseudogilding

Abstract

Adherent films of the gold-colored metal sulfides pyrite (FeS2), pyrrhotite (Fe, -$), and chalcopyrite (CuFeS2) are known to exist on metallic and nonmetallic artifacts of Classic times. Consideration of the action of bacterial consortia containing sulfate reducing bacteria (SRB) during microbiologically influenced corrosion (MIC) and analyses of appropriate trajectories on Eh-pH stability diagrams indicate that these films can be understood either as a result of natural processes over archaeological times or of deliberate pseudogilding. Methods of discriminating natural sulfide deposition from pseudogilding are suggested. 9

The Use of Mineralogical Data in Interpretation of Long-Term Microbiological Corrosion Processes: Sulfiding Reactions

“…Deposits on admiralty brass (60% Cu/39.25% Zn/ .75%Sn) exposed to seawater, 2500x. (Daubree 1862;de Gouvernain 1875;McNeil and Mohr 1993), and orickite (hydrated chalcopyrite) has also been observed (Tribe 1998). While chalcopyrite films can be formed abiotically in high sulfur concentrations (Cuthbert 1962), chalcopyrite and most other copper sulfides are not generally found as products of abi- Roseboom 1966;Craig and Scott 1976;Putnis 1977;Evans 1979).…”

“…Recent work indicates that on continued exposure to SRB mackinawite alters to smythite (Fe9Sn), to greigite (Fe3S4), to pyrrhotite (FeSl+x), and finally to pyrite (FeS2) (McNeil and Little 1990). SRB in thin biofilms on pottery surfaces (Duncan and Ganiaris 1987;Heimann 1989) and silver (McNeil and Mohr 1993) can produce pyrite films from iron-rich waters. SRB can produce pyrite from mackinawite in contact with elemental sulfur (Berner 1969), and thermodynamic analyses indicate that elemental sulfur can coexist with various iron sulfides.…”

The Mineralogy of Microbiologically Influenced Corrosion

“…Copper oxides are readily converted to copper sulfides [83]. Biomineralogy of copper sulfides has been studied for over a century [84e91].…”

“…Biomineralogy of copper sulfides has been studied for over a century [84e91]. Detailed kinetics of individual reactions is not fully understood, and the consequences for corrosion depend on many factors, including mineral morphology and variations of redox and pH with time [83]. The general phenomenology can be understood by the following approach.…”

“…Microbial consortia that include SRB produce anoxic, sulfide-rich environments in which the conversion of copper to copper sulfides is thermodynamically favored at a concentration of 10 À2 M total sulfur. In long-term corrosion where waters contain significant iron, CuFeS 2 (chalcopyrite) is a common product [83]. One would expect a layering effect with CuS (covellite) on the outside and Cu 2 S (chalcocite) next to unreacted copper metal.…”